If we have a lot of simultaneous sessions we really shouldn't send the full
list of active sessions with each PropertyChanged message for user and seat
objects, as that can become quite substantial data, we probably shouldn't dump
on the bus on each login and logout.

Note that the global list of sessions doesn't send out changes like this
either, it only supports requesting the session list with ListSessions().

If cients want to get notified about sessions coming and going they should
subscribe to SessionNew and SessionRemoved signals, and clients generally do
that already.

This is kind of an API break, but then again the fact that this was included
was never documented.